I've noticed that one of my machines here suffers from the "time going
backwards problem" and so started thinking about the x86 solution.

I've come to the conclusion that it has a hole which could cause it
to return the wrong time in one specific case:

- in do_gettimeofday(), we disable irqs (read_lock_irqsave)
- the ISA timer wraps, but we've got interrupts disabled, so no update
of xtime or jiffies occurs
- in do_slow_gettimeoffset(), we read the timer, which has wrapped
- since jiffies_p != jiffies, we do not apply any correction
- our idea of time is now one jiffy slow.

Further more, while do_gettimeofday() is still within the
read_lock_irqsave, we spin_unlock(&i8253_lock) in do_slow_gettimeoffset()
and _re-enable_ interrupts! This means when we later read xtime, we're
doing it with interrupts enabled.